The present invention relates to the use of techniques for packet transmission in acknowledged mode in radio communications systems.
The invention especially relates to the techniques which invoke automatic repetition of the packets or blocks for which the transmitter observes, according to acknowledgement information returned by the receiver, that they have not been correctly received (methods of ARQ (Automatic Repeat request) type).
The application of these techniques to mobile radio communications presents particular features because the errors affecting the blocks transmitted on the radio channel are not decorrelated. These errors are manifested by channel fades, having an average duration decreasing with the speed of the mobile station (Rayleigh fading). For a slow-moving mobile station, such channel fade typically lasts of the order of one to several hundred milliseconds.
An acknowledgement message returned by the addressee of the blocks, for example in response to a request from the transmitter (polling), usually relates to a series of transmitted blocks. When this series ends in a channel fade, the chances are high that one or more subsequent blocks have also been incorrectly received. The transmitter repeats the blocks for which it has been able to observe the incorrect reception, but it has to wait for the next acknowledgement message to be up to repeat these subsequent blocks. This wait leads to delays in the transmission of the blocks. The transmission delay, however, is often a critical problem in this type of system.
One object of the present invention is to propose an effective method of transmitting data in packet mode on radio links. Another object is to reduce the transmission delays which may result from the ARQ mechanisms employed in such systems.